(1) Field of the Invention
The present invention relates to a method for forming a coating film on a plastic substrate.
(2) Description of the Related Art
Coating of plastic substrates has heretofore been carried out by spray coating methods such as air spraying, airless spraying and the like. However, in order to save energy and to reduce harmful wastes emitted into the environment, electrostatic coating, which achieves high coating efficiency, is coming into wide use.
Because plastic substrates in general have high electrical resistivity (usually about 1012 to 1016 Ω/square), it is extremely difficult to apply a coating composition directly to a plastic surface by electrostatic coating. Therefore, electrostatic coating is usually carried out after the plastic substrate itself or surface thereof is made electrically conductive.
For example, before a plastic substrate is electrostatically coated, an electrically conductive primer is applied to provide the substrate with electrical conductivity. The electrically conductive primer to be used usually contains a resin component and an electrically conductive filler.
The electrically conductive fillers usually used are particles of electrically conductive carbon, metal, electrically conductive metal oxides and the like. The particle form of the electrically conductive fillers is usually powdery, acicular, fibrous or spherical.
Carbon powder or carbon fiber, which has a low specific gravity, is preferable as the electrically conductive filler in that the conductive effect can be obtained by the use of a small quantity thereof. However, there are some problems that need to be solved. The whiteness, or the lightness, of the coating film is decreased, so that the color tone of any overcoat is affected.
Although metal powders or metal fibers have high electrical conductivity, the quantity thereof required is large, decreasing the whiteness of the coating film or the stability of the coating composition, because the particles need to be in contact with one another in order to form electrically conductive paths through the coating film.
Likewise, though acicular or fibrous conductive fillers are favorable for providing high electrical conductivity, they are scattered through the air when the coating composition is produced or the coated plastic substrate is recycled, gravely affecting the environment and the human body.
Problems also arise when a spherical conductive filler is used. The amount thereof required is large, resulting in an unstable coating composition or involving high cost.
Japanese Unexamined Patent Publication No. 1983-64164 discloses a method of coating the surface of a plastic substrate with an undercoat containing a volatile conductive substance, and electrostatically coating with a topcoat composition the undried undercoat film having the volatile conductive substance remaining therein, followed by drying both coating films by heating. This method has made it possible to shorten the coating process and to reduce cost. However, the undercoat containing a volatile conductive substance has problems in view of the recent regulation on volatile organic compounds (VOCs).
Japanese Unexamined Patent Publication No. 2001-311047 proposes an electrically conductive coating composition containing a particular sulfonium salt compound which is capable of rendering plastic materials compatible with electrostatic coating and does not affect the hue of an overcoat in the formation of a multilayer coating film. The sulfonium salt compound, however, adversely affects the environment when coating films are baked or when coated plastic products are recycled, and the whiteness of the coating films is not sufficient.
Japanese Unexamined Patent Publication No. 2001-29873 discloses a method for forming a coating film wherein, after a plastic surface is coated with a colored coating composition containing titanium white pigment and fine flaky mica coated with a metal oxide, a pearl-colored coating composition and a clear coating composition, the resulting three coating film layers are subjected to simultaneous baking. However, it is impossible to obtain from the colored coating composition, a colored coating film having a surface electrical resistivity value of less than 109 Ω/square, the value at which electrostatic coating is possible. The lightness of a multilayer coating film thus obtained is not sufficient.
Japanese Unexamined Patent Publication No. 1998-296171 discloses a method for coating a plastic material wherein the surface of the plastic material is coated with an aqueous primer to form a primer coating film, the primer coating film being then coated with a colored coating composition to form a colored coating film, the colored coating film being coated with a clear coating composition to form a clear coating film, and followed by baking the three-layer coating film. In this method, however, the primer usually has a high water content of 50% or more by weight, and part of the water thereof may remain in the coating film, resulting in low water resistance. In addition, the lightness of the multilayer coating film thus produced is not satisfactory.